The fuel cells of a fuel cell power plant operate on gaseous fuel. Where the raw fuel is itself gaseous it is preheated in a heat exchanger as it enters the fuel processing system. The heat is supplied by gaseous effluent from the reformer. These heat exchangers are designed for counterflow heat transfer whereby the maximum heat transfer is achieved with minimum surface.
The various components of the fuel processing system must be preheated before operation of the plant. Conventionally an inert gas is introduced into the system where heat is supplied by firing in the reformer. This inert gas is recycled through the system, being cooled in a heat exchanger to prevent overheating of some of the low temperature components. The same heat exchanger as is used for heating the fuel is used to both cool this gas upstream of the low temperature components, and to partially reheat the gas as it is recycled to the high temperature components. The previously described counterflow relationship is ideal for this since it achieves maximum efficiency of the heat exchanger.
On occasions, however, it is desirable to operate a fuel cell plant with a liquid raw fuel feed. If such fuel is introduced in counterflow relationship with the high temperatures encountered at the reformer outlet, there is potential for fuel cracking and heat exchanger fouling. Conventional systems have used a second heat exchanger to perform this duty.